One of the more important properties of any structural product such as panels or lumber products is the modulus of elasticity (MOE). Obviously other properties such as modulus of rupture (MOR), tensile strength, compression strength, etc., also are factors but the MOE for many applications is the critical characteristic.
Composite wood products having high strength in particular a high MOE have been sought after for many years. The Barnes U.S. Pat. No. Re. 30,636 issued June 2, 1981 (a re-issue of U.S. Pat. No. 4,061,819 issued Dec. 6, 1977) it is believed is the first recognition that the strength of composite product could be significantly increased. This patent teaches that strength is density dependent, i.e. the higher density generally the higher the strength of the product for the same starting materials and that by changing the starting materials, particularly by increasing the length of the strands used, the strength to density ratio could be significantly improved. This system utilized wood elements in particular strands having lengths of at least 12 inches, width of 0.05 to 0.25 inches and thickness of 0.05 inches to 0.5 inches formed by slicing and then clipping. The widths and thicknesses specified in this patent are the opposite to what one would normally define as width and thickness in a waferizing operation or in producing a clipped veneer strand, i.e. normally the thickness is determined by the thickness of the veneer and the width is determined by the spacing between adjacent clips whereas with the Barnes patent the width of the strand is the thickness of the veneer and the thickness of the strand is the spacing between clips.
In any event the strength characteristics of the products produced using the Barnes teachings produced wood products having MOE's in the order of up to about 2.2 mm psi at a wood density over 35 lb/cu.ft..sup.3 using strands 24 inches long.
The Holman U.S. Pat. No. 4,255,477 issued Mar. 10, 1981 also relates to panel or strand lumber products having improved strength. This patent teaches the use of wood elements, in particular `boat shaped wood strips` having lengths in the order of 8 to 12 inches and according to his examples was able to successfully produce a board product having an MOE of 1.7 mm psi at a board density of about 48 (wood density probably around 44 lbs/cubic ft.) and a second product having an MOE of 1.6 mm psi at a slightly higher density. Holman attributes his `high` strength products to the use of a variety of particularly shaped and sized elements that are substantially boat shaped in axial cross section and had lengths up to about 12 inches.
More recent patents of Barnes namely U.S. Pat. Nos. 4,610,913 and 4,751,131 issued Sept. 9, 1986 and June 14, 1988 describe a high strength panel product and a high strength lumber product respectively made from long wafers (lengths 6 to 12 inches and longer). Prior to these teachings, panels or lumber products made of wafers always employed short wafers in the order of up to 4 inches in length and the art clearly taught that extending the length of the wafer brought no significant benefits in increased strength and thus for many years, until the more recent teachings of Barnes, it was believed that if wafers were used to produce composite wood products extending the length of the wafer beyond about 3 inches was of no merit.
The latest Barnes patents teach that as the wafer length is increased the MOE to density ratio for a panel product can be increased significantly and with a wood density in the order of about 35 lbs/cubic ft. panels having MOE's in the order of 1.6 mm psi could be obtained using wafers over about 12 inches in length and that with higher wood densities and longer lengths the MOE could be further increased.
The lumber product described in U.S. Pat. No. 4,751,131 produced a lumber product having a wood density in the range of about 35-40 psi using wafers over 8 inches long having MOE's in the range of about 1.6 to 1.7 mm psi which was significantly higher than the MOE's for composite lumber products produced in the prior art.
Laminated veneer lumber is usually made by peeling veneer from a log by rotating the log and peeling a ribbon of veneer from the surface of the log. Generally the resulting veneer is eight foot long measured in the grain direction, i.e. axially of the log being turned and has a thickness in the order of about 0.1 inches. A laminated veneer lumber is produced by laying up a plurality of layers of such veneer with the grain extending substantially parallel on each of the layers and securing the layers together under heat and pressure using an adhesive such as a phenol formaldehyde. The veneer sheets, when they are peeled, generally have checks (lathe checks) extending in the grain direction. These checks are formed by the peeling operating itself.
Laminated veneer products made as above described generally have MOE's up to about 2.1 mm psi at wood densities in the range of about 35 lbs/cubic foot. Repairing of the lathe checks by addition of extra resin has been attempted but no one has reported that this contributed significantly to increasing the MOE of the laminated veneer product made from such repaired veneers.